The present invention relates to a logic circuit which is capable of adjusting a switching threshold in accordance with the voltage level of applied input logic signals.
Digital logic circuits are sometimes arranged such that a first logic circuit operates with one supply voltage, e.g., 3.3 volts, while another circuit supplying input logic signals to the first logic circuit operates with a different supply voltage, e.g., 1.8 volts. In such a case, the logic signals supplied to the first logic circuit are incompatible with the switching threshold of the first logic circuit, which is set for applied input signals of 3.3 volts, causing erroneous and undesired switching operations.
A simple way of adjusting a switching threshold of a logic circuit to handle input logic signals of different voltage levels is needed.
The present invention provides a simple way of adjusting the switching threshold of a logic circuit in accordance with the signal level of input logic signals which are applied to it.
The invention provides a logic circuit with a voltage level detector stage which detects the supply voltage of a circuit supplying applied input signals, and an adaptive threshold stage which, in response to the output of the detector stage, selects a switching threshold level for switching operations in response to the applied logic input signals.
The adaptive threshold stage may be formed as a CMOS inverter circuit in which certain transistors are turned on or off in response to the output of the detector stage to thereby alter the inverter switching threshold.
These and other features and advantages of the invention will be more clearly seen from the following description of the invention which is provided in conjunction with the accompanying drawings.